124 ORGANISED FLUIDS. 



described, between which they are intermediate in situation, 

 as in structure, participating in the characters of the false or 

 true raucous membranes more or less, according to the pre- 

 ponderance of either of these classes. The membranes of 

 this division may be called mixed, and those of the mouth 

 and nose may be regarded as typical. 



Now, however useful for the purposes of description the 

 above classification may be, it must still be remembered that 

 it is, to a very considerable degree, arbitrary ; the membranes 

 which we have described as false mucous membranes belong 

 rather to the skin than to true mucous structure, while the 

 mixed membranes exhibit only the gradual transition from 

 the external skin to the internal true mucous membrane: 

 thus, strictly speaking, there is but one class of mucous mem- 

 branes, and that the true. Corresponding with the differ- 

 ences which have been pointed out as characteristic of the 

 three classes of mucous membranes, there are others apper- 

 taining to the mucus secreted by each of these orders of 

 membranes, and which arise from their diversity of structure, 

 and which serve to distinguish the mucus of the one class 

 from that of each of the other classes. 



1st. The mucus proceeding from true mucous membranes 

 is viscous and alkaline, containing, imbedded in its substance, 

 numerous spherical, semi-transparent, and granular corpuscles 

 of about the ^Vo of an inch in diameter (see Plate XL 

 Jig. 1.)*, having a somewhat broken outline, as well as oc- 

 casionally epithelial cells more or less cuneiform, and some- 

 times provided with cilia?. These corpuscles are for the most 

 part nucleated, they are not at first soluble in water, but 

 swell up in that fluid to two or three times their former 

 dimensions (see Plate ~KL.jig. 3.), and, like the white globules 

 of the blood to which they bear the closest possible resem- 



* A law having reference to size, and the importance of which will be 

 hereafter demonstrated, may here be announced. It is that the several 

 structures, especially the corpuscular ones, entering into the composition 

 of the animal organization, bear a near relation of size the one to the 

 other. 



